The Universal Serial Bus (USB) is described in its current release in the Universal Serial Bus Specification Revision 1.1 and is envisioned as an industry standard for the connection of all peripherals to a personal computer or workstation. USB seeks to replace the various specialized connectors and communication formats utilized with keyboards, mice, joysticks, multimedia inputs and outputs, printers, external disk drives, external CD-ROM drives, modems, parallel ports, serial ports, network connections, and virtually any other type of peripheral, with a single universal cable and connector type and a common communication format. Some of the other features of USB are: a true plug-and-play architecture requiring no user configuration, flexibility, and low cost.
USB is a complete break from the past--it provides no backward compatibility with previous communication interfaces. But many new users of USB-equipped computers have significant investments in non-USB peripherals such as parallel cable-interfaced printers. Rather than junking these legacy peripherals, many users would prefer to somehow utilize non-USB peripherals with USB-equipped computers. Several vendors have addressed this need by offering USB bridges, which convert USB signals to a legacy format and vice-versa.
The problem with using a USB bridge is that a substantial latency penalty is imposed by the USB hardware and software each time an instruction or data is transferred across the bridge between the USB bus and the peripheral. When a sequence of commands is conducted over the USB bus, the delay accumulates as each individual command is sent separately via a USB transaction over the Universal Serial Bus. As a result, long sequences of operations sent to the USB bridge create latency problems.